Diodes are electrical components designed to conduct an electric current in one direction and resist it in another. A fast diode is simply a diode that is able to switch from conducting to resisting as quickly as possible. Diodes fall into three basic categories: the standard diode, the soft diode and the fast diode. When a diode needs to switch between conducting and resisting, there is a built-up charge that needs to be dealt with, and the differences between the three types of diodes have largely to do with how they deal with this charge.
This is done with a positive-negative (P-N) semiconductor junction, which is where the two individual pieces of the diode connect, separated by a thin material. When a diode makes the transition from conducting to resisting, the built-up charge within the junction must be dispelled before the diode is effectively blocking voltage. This is known as the "reverse recovery time" of the diode, and the length of time it takes for a diode to move from conducting current to opposing current is measured in hundreds of nanoseconds for standard and soft diodes.
A fast diode is the quickest but most volatile way of disposing of this extra charge. This effectively gets rid of the charge in the fastest, most violent and forceful way possible, with little regard for how the current will temporarily disturb the workings of the system. The time it takes for this to occur is measured in tens of nanoseconds for a fast diode, rather than the hundreds of nanoseconds it takes the standard and soft diodes. The dispelled charge of a fast diode can cause various high frequency (HF) and radio frequency (RF) components to be generated within the circuit. These components can be dampened with resistors in series with small capacitors to dampen any interruption of the circuit, but they do nothing to help increase the reverse recovery time of the diode.
Fast diodes are built to increase the speed of the reverse recovery time at the expense of the fluidity of the action. Soft recovery diodes focus on limiting the generation of unnecessary harmonic components at the expense of speed. Although soft diodes do not focus on the speed of the reverse recovery time, they still are much faster than standard diodes.